Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses for eye glasses and other optical instruments, as well as glazing used on skyscrapers, also employed shatter-resistant, transparent plastics. The ligher weight of these plastics compared to glass is a further advantage, especially in the transportation industry where the weight of a vehicle is a major factor in its fuel economy.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, and good dimensional stability, as well as being self-extinguishing and easily fabricated.
While transparent plastics provide the major advantages of being more resistant to shattering and lighter than glass, a serious drawback to their use is the ease with which they mar and scratch due to everyday contact with common abrasives: dust, cleaning solutions, etc. Continued scratching and marring eventuates in impaired visibility and poor aesthetics, and often requires replacement of the glazing or lens. Another drawback of some transparent plastics is that prolonged exposure to ultraviolet radiation causes discoloration due to the chemical breakdown of the polymer structure. For example, polycarbonate, mentioned above, is particularly susceptible to discoloration from ultraviolet light exposure.
Many attempts have been made to improve the abrasion resistance of transparent plastics. U.S. Pat. No. 3,708,225 (Misch et al.), U.S. Pat. No. 3,976,497 (Clark), U.S. Pat. No. 3,986,997 (Clark), U.S. Pat. No. 4,027,073 (Clark), U.S. Pat. No. 4,159,206 (Armbruster et al.), and U.S. Pat. No. 4,177,315 (Ubersax), for example, describe sratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium such as alcohol and water. Copending U.S. application Ser. No. 964,910, filed Nov. 30, 1978 now abandoned, discloses another abrasion-resistant coating composition which has been found to be highly desirable as a protective finish for plastic, as well as metal or metallized substrates.
A particularly significant area of application for these coatings is in the glazing and optical lens industry. Because a high degree of optical clarity is required, coatings which show flowmarks, dirtmarks, or other marks which may impair visibility, are undesirable. In copending U.S. application Ser. No. 964,911, filed on Nov. 30, 1978 now U.S. Pat. No. 4,277,287, it is disclosed that the addition of a small amount of a polysiloxane polyether copolymer to the coating compositions disclosed therein eliminates the occurrence of undesirable flowmarks and the like, as well as providing other improvements in the hard resistant coating.
U.S. Pat. No. 4,299,746 (Frye), incorporated herein by reference, discloses a further improvement in silicone resin coating compositions used on plastics, namely that certain ultraviolet light absorbing compounds effectively extent a coating's service life and resistance to discoloration when incorporated into silicone resin coating compositions.
It has now been discovered that the service life and resistance to delamination and discoloration of silicone resin coatings can be increased by the incorporation of an ultraviolet light absorbing compound, or a combination ultraviolet absorbing compound and an ultraviolet radiation stabilizing agent, into the primer coat normally employed to enhance adhesion of silicone resins to a particular substrate. Modifying the primer in this way does not significantly alter the desirable adhesion characteristics, optical clarity or abrasion resistance of the coating compositions. Moreover, the absorber-stabilizer combination produces prolonged life and increased resistance to delamination and discoloration.